Transducer



1958 H. w. PARKER TRANSDUCER Filed Nov. 29. 1954 \INVENTOR HENRY W. PARKER KILOVOLTS PER MM.

204% mma mmzro m a 2,864,899 lcfi Patented Dec. 1958 TRANSDUCER Henry W. Parker, Flushing, N. Y.

Application November 29, 1954, Serial No. 471,807

1 Claim. (Cl. 179111) The present invention relates to transducers converting electrical energy into sound waves.

In the production of sound waves by electrical means, resort has been made to the use of piezo-electric, electromagnetic and electrostatic types of transducers. The piezo-electric transducer, although of lightweight, does excite a diaphragm cone which has an unsatisfactory transfer of energy from the diaphragm to the air medium in terms of spurious frequencies and efficiency. The generally accepted electromagnetic transducer is likewise low in its transfer efficiency, poor in the transduction of high audio frequencies unless dual sizes are employed, unsatisfactory in the generation of spurious frequencies, heavy in weight, occupies too much space and the electromagnetic transducer utilizes critical materials in its construction. The electrostatic type of transducer presently in use in the art, has recognized merit; but its shortcomings limit its utility to tweeters, in which the excursion of the diaphragm is small because of the high audio frequency and limited excitation is employed to minimize the amount of distortion.

Accordingly, it is the object of the present invention to provide an improved method and means for converting electrical energy into sound waves. Specifically, it is within the contemplation of the present invention to provide an eflicient transducer having high fidelity in the production of sound from the exciting electrical wave from the infrasonic to the ultrasonic regions of audible spectrum, by a light weight small size embodiment of the present invention made of non-critical materials.

Among the many applications of the present invention, without limitation, are earphones and loudspeakers as well as more general applications including scientific instruments which require a linear relation between sound pressure and the exciting voltage.

Certain objects of the present invention are realized by the provision of an interdigitated structure of the mobile diaphragm and its two associated polarized outer electrodes on either side of the mobile electrode, which interdigitated structure allows a large diaphragm excursion without modifying that part of the electric field which pulls the interdigitated electrodes together in a direction normal to the surface of the mobile diaphragm. The pull of the polarized electrodes on the two opposite sides of the mobile electrode are vectorially oppositely directed, so the vector sum of the pulls is a simple substraction. Utilization of this elementary fact is the main object of the present invention since it permits a linearity to exist between the exciting voltage and the sound pressure, as hereinafter explained.

A further object of the present invention is realized by uniform excitation of the mobile diaphragm over its large area, without the distortions and spurious tones excited in the mobile diaphragm by old art forms of electrostatic transducers whose sound energy output was not proportional to the excitation voltage squared.

Certain objects of the present invention are realized by the provision of a diaphragm having small weight per clearance holes in the cooperating outer electrodes.

unit area so that when it is driven by the electric force field, resulting from excitation of the outer electrodes, the uniform force per unit area and the low inertance per unit area will provide good high audio frequency performance as a transducer.

There is one preferred form of my invention shown in Figs. 1 and 2, which I choose to describe in detail for purposes of illustration. This embodiment is close to the ideal electrostatic transducer in providing nearly exact linearity between sound pressure and the applied electric signal voltage over a Wide range of sound power, as will be described later.

The various features and aspects of the present invention, including further objects and practical applications will be more fully appreciated upon reference to the following description of the presently preferred embodiment, when taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a diagrammatic showing of a studded diaphragm and honeycomb outer electrode structure of an electric energy to sound pressure transducer of the present invention, devised to provide a loudspeaker giving sonic response from the infrasonic to the ultrasonic regions of the audible spectrum.

Fig. 2 is a diagrammatic showing of a section of the transducer of Fig. 1.

Fig. 3 is a schematic circuit representation showing the electric exciting circuit applied to the outer electrodes and the polarizing voltage applied to the insulated diaphragm.

Fig. 4 is a curve showing the square law of pressure plotted against the electric field intensity, for the constant efi'ective air gap spacing of the straight sided projections on the mobile diaphragm, 4 of Figs. 1 and 2, and the straight sided holes in the outer electrodes, 3 and 4 of Figs. 1 and 2, in registration with the said projections on the mobile diaphragm of this invention.

The expression mobile diaphragm means the stretched metallic membrane, 4 of Figs. 1 and 2, which moves under action of forces of the electric field when an electric signal voltage is superimposed on the electrostatic voltage polarizing the cooperating electrodes 3 and 4 and 5, as shown in Fig. 3. The expression straight sided projections means that the projections on the mobile diaphragm may be straight sided cylinders, straight sided prisms or straight sided corrugations, having the straight sides normal to the plane of the mobile diaphragm. In the description, the straight sided cylinder will be discussed only for purposes of illustration. The expression straight sided holes similarly means round, rectangular or grille shaped openings in the cooperating outer electrodes 3 and 5 of Figs. 1, 2 and 3. The words outer electrodes means the two fixed electrodes with holes in registration with the projections of 4. The outer electrodes are charged to a static potential to set up an electric field between 3 and 4 and 5 and 4. The outer electrodes are separately insulated and are spaced from 4 to provide an air gap which contains the important electric field flux when the electrodes are polarized. The phrase partially inserted interdigitated electrodes means that the regular array of straight sided projections on the mobile diaphragm are partially inserted into the registrated straight sided holes in the outer electrodes 3 and 5, so there exists a constant fringing electric field gap, independent, within wide limits, of the instant position of the mobile diaphragm 4, as described in Fig. 2. The expression normal component of fringing field flux means the component of the electric field perpendicular to the plane of the mobile diaphragm which emerges from the ENDS of the straight sided projections on the mobile diaphragm and terminate on the straight sides of the It is only the normal component of electric field flux which produces a force tending to move the mobile diaphragm, held in constraint by clamped edges. The parallel component of the electric field flux is redundant and this redundancy is minimized by partially inserting the interdigitated electrodes. The expression clearance holes in the outer electrodes means that an insulating air gap must be maintained to avoid arc-over between the mobile diaphragm and the cooperating outer electrodes.

For a more thorough and complete understanding of the scope of the present disclosure, some of the concepts of the electric field should be considered.

In the discussion, it will be shown how a linear relation between excitation voltage and sound pressure is obtained by applying a polarizing potential to the insulated mobile diaphragm and an A. C. excitation voltage to the outer electrodes. As shown in Fig. 3, the A. C. voltages applied to the opposite outer electrodes are equal in amplitude and 180 out of phase. By a subtractive process taking place in the opposed pulls of the diaphragm, a linear relation of pressure versus applied signal voltage results.

Equation 1 expresses the tension per square centimeter in terms of the volts difference V, and the air gap separation distance s, between the partially inserted interdigitated electrodes measured in a direction normal to the plane of the mobile diaphragm. The tension is expressed as dynes per square centimeter and is regarded as a pressure p in this discussion.

The above equation is used in plotting the curve of Fig. 4 valid only for a constant electric field gap and distinctly not applicable to a planar diaphragm whose movement alters the gap.

For high fidelity production of sound, a linearity between pressure and the applied signal voltage is required, because the intensity of sound depends on the pressure of the sound wave squared as shown in Equation 2, where p is the maximum value of the excess pressure of the sound wave in dynes per square centimeter; c is the celerity of the sound wave taken as 331x centimeters per second; p is the air density at sea level taken as 1.27 milligrams per cubic centimeter and I is the intensity of the sound expressed in ergs per square centimeter.

Examination of Equation 1 shows that if s, the gap distance, is kept substantially constant, the pressure p depends on the voltage squared, and hence, by Equation 2, the sound intensity will vary according to V*, the voltage to the fourth power. Only by high frequency operation at low audio power levels is such an electrostatic device capable of giving acceptable performance.

The maximum tension that can be supported in air at sea level is determined by the maximum volts per centimeter allowable. At 3000 volts per centimeter, the maximum tension is 400 dynes per square centimeter, which on substitution in Equation 2, shows about 0.2 milliwatt per square centimeter of sound radiating surface. This power can be improved by certain electrical insulation processes which will allow a greater field strength.

In the present invention, the gap s is maintained constant by the use of a studded diaphragm in register with holes in the outer electrodes, see Figs. 1 and 2, thus permitting a large diaphragm excursion required for low audiofrequency operation.

Referring now to Fig. 3 let V be the D. C. polarizing voltage applied to the insulated mobile diaphragm electrode, and let there be an A. C. signal voltage from a mid-tapped source of excitation applied to the outer electrodes, as shown schematically in Fig. 3, then the net pressure exerted by the diaphragm will be the difference Examination of the above equation, reveals the conditions for high quality production of sound from an electrostatic transducer, namely, the polarizing voltage and the air gap spacing must be substantially constant, the signal e from mid tap to the outer electrode on one side must be not only equal in amplitude but also out of phase with the signal e applied to the other outer electrode, and further, the signal 2 should not exceed the polarizing voltage V Under these conditions, the excess pressure of the sound wave is a linear function of the signal voltage and a high fidelity electrostatic transducer is the result.

The amplitude of motion of a longitudinal wave of sound is useful in calculating the amplitude of motion of the mobile diaphragm. In the following equation of the mobile diaphragm. In the following equation, Equation 4, the amplitude A of the longitudinal motion is expressed in terms of the pressure p in dynes per square centimeter, the air density p, and the celerity c and the angular velocity w in radians per second.

By choice of a low inertance diaphragm, weighing only a few milligrams per square centimeter, the diaphragm amplitude will approach A in the limit, and a very emcient and high fidelity transducer will be commercially available by using the new principles of the present invention.

Referring now to Fig. 1, there is shown a diagrammatic section of an electrostatic loudspeaker which provides an output sound pressure proportional to the first power of the applied signal voltage. The mobile d aphragm, studded with an array of studs in register with holes in the outer electrodes 3 and 5, is attracted by the tension of the electric field established between the studded diaphragm and the outer electrodes. The motion of the diaphragm is towards the outer electrode which provides the greater field strength. For a diaphragm motion of less than one millimeter, which is large for an electrostatic transducer, the gap between electrodes 4 and 3 and between electrodes 4 and 5 is substantially constant. Since only the fringe field" acting on the ends of the studs of diaphragm 4, and on the edges of electrodes 3 and 5 facing the diaphragm, is effective in pulling the electrodes together under the influence of an applied electric field, it can be seen that because of the coaxial structure of the studs and the holes in 3 and 5, the fringe field remains substantially constant when the diaphragm makes an excursion. This fact of keeping the gap s constant, permits us to say, in accordance with Equation 1, that the sound pressure varies as the -voltage squared. By placing a polarizing voltage V as a constant D. C. potential between the electrodes 4 and 3 and also between electrodes 4 and 5, and then superimposing an A. C. signal voltage from a mid-tap transformer, as shown in Fig. 3, on the outer electrodes, there is, as explained in the derivation of Equation 3, a linear relation obtained between the resultant pressure and the applied signal voltage.

In Fig. 1 the structure is composed of a face plate 1 which is grounded to the cabinet of the loudspeaker. The holes in the face plate are for passage of the sound through the holes 2. The electrodes 3, 4 and 5 are spaced and supported by plastic type of insulators 6 and 7 and held together by through bolts 8. The lugs 9 and 1t) and 11 are for electrical connection to the electrodes 3, 4 and 5. The shape of the lug strip 11 allows tightening of the diaphragm when the bolts 8 are tightened.

Fig. 2 is an enlarged section of Fig. 1, showing the detail of the hollow bosses in the mobile diaphragm 4. These bosses are arranged in a regular array to mate with the holes in the outer electrodes 3 and 5 so that the air gap between the electrodes is larger than a minimum allowable gap, to ensure against arc-over.

In Fig. 3 is shown the schematic of a preferred circuit for applying signals equal in amplitude but opposite in phase to the outer electrodes as required in the production of undistorted sound from the electrostatic transducer of the present invention. 3, 4 and 5 have already been described for Fig. 1. The battery 16 of Fig. 3, is representative of a source of polarization potential which applies a D. C. potential between electrodes 3 and 4 and between 4 and 5. The polarity is unimportant because the pull between the electrodes is independent of the polarity, but depends on the intensity of the field only. The transformer 15 has a mid-tapped secondary winding for the purpose of meeting the necessary condition that the voltages applied to the outer electrodes by the signal be 180 out of phase and preferably equal in a balanced design of the electrostatic transducer of the present invention. The grounding 17 is shown at the mid-tap of the transformer, but it could be connected to the electrode 4 thus making it unnecessary to insulate electrode 4 from ground.

The curve shown in Fig. 4 is a plot of Equation 1 based on the assumption that s is kept constant. In the example shown, Q is the quiescent point of pull caused by the D. C. polarizing potential. QP is a measure of that pull, equal in the direction of each opposed outer electrode. If we now upset this equilibrium by applying a signal voltage of equal amplitude but opposite phase to the outer electrodes, we obtain a pull AA on one electrode and BB on the other so the diaphragm has a net pull in the direction of the outer electrode pulling with a force AA. Being opposed vectorially, the forces are resolved by a simple subtraction of their pulls on the diaphragm. As described in Equation 3, the linear dependence of pressure on signal voltage is obtained by the prior art method of applying the signal voltage to both sides of the new mobile diaphragm of the transducer of the present invention, in the manner described in this disclosure.

What I claim is:

A substantially constant air gap, three electrode, electrostatic loudspeaker comprising an insulated studded diaphragm electrode, held in constraint by frame clamped edges, midway between two opposite separately insulated cooperating foraminated planar outer electrodes fixed parallel to the plane of the diaphragm, the said diaphragm studded on both sides with a plurality of straight sided right prismatic projections partially inserted in registered straight sided clearance holes in the said foraminated outer electrodes.

References Cited in the file of this patent FOREIGN PATENTS 251,349 Great Britain Apr. 30, 1926 326,602 Great Britain Mar. 2, 1930 387,546 Great Britain Feb. 9, 1933 

